The present invention relates to a centrifugal flow fan and fan/orifice assembly for moving air. In particular, the invention relates to a high efficiency centrifugal flow fan and fan/orifice assembly that provide improved fan performance at high static pressures.
Centrifugal flow fans are used to change the movement of air from a direction arranged generally parallel to the axis of fan rotation to a direction arranged generally radial (perpendicular) to the axis of fan rotation. FIG. 5 is a diagram showing an example of a centrifugal fan and orifice assembly used for this purpose. The orifice includes a bell-shaped inlet bulkhead 110 defining an inlet orifice 111 for introducing air into the fan. A centrifugal fan 112 is provided downstream of the inlet and is driven by a motor 120 to draw air into the inlet orifice in a direction generally parallel to the axis of fan rotation and force the air radially outwardly from the blades 113 of the fan. The fan includes a shroud 114 to help direct the air centrifugally out of the fan body.
In the assembly depicted in FIG. 5, the structure of the fan blades is such that the diameter, d, of inlet orifice 111 is usually less than 70% the diameter, D, of fan 112. As a result, motor 120 blocks a significant portion of inlet orifice 111, causing a substantial pressure drop through the inlet orifice. The fan must work harder to compensate for this pressure drop. While the pressure drop through the inlet opening could be reduced by reducing the size of the motor used to drive the fan, this would in most cases limit motor power and speed, and thus air flow.
Additionally, the leading edge 113L of each fan blade is radially spaced from the outer surface of motor 120 to allow air to enter the body of the fan blades without impediment. However, this degrades the overall performance of the fan, since the radial spacing at the base region of the fan blades is essentially wasted space. That is, the air entering the fan is already on the body of the blades and is being redirected centrifugally by the time it reaches the base region of the blades. Accordingly, the radial spacing at the base region of the fan blades does not greatly contribute to the entry of air into the body of the fan blades, and the absence of fan blade structure in this region takes away from the ability of the fan to push the air centrifugally out of the fan body.
It would be desirable to reduce the pressure drop through the inlet orifice due to the presence of the motor housing, and also increase the overall efficiency of the fan without impeding the flow of air into the body of the fan blades.